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CHAPTER 13 s USING DATASETS AND DATA ADAPTERS
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This would be ideal in terms of performance, but it d be feasible only if the data you needed were limited to these specific rows in this particular sequence. However, if you were building a more elaborate system, it might be better to pull all the data once from the database (as you do here) and then filter and sort it in different ways. ADO.NET s rich suite of methods for manipulating datasets and their components gives you a broad range of techniques for meeting specific needs in an optimal way.
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s In general, try to exploit SQL, rather than code Visual Basic .NET procedures, to get the data you need Tip from the database. Database servers are optimized to perform selections and sorts, as well as other things. Queries can be far more sophisticated and powerful than the ones you ve been playing with in this book. By carefully (and creatively) coding queries to return exactly what you need, you not only minimize resource demands (on memory, network bandwidth, and so on), but also reduce the code you must write to manipulate and format result set data.
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The loop through the second data table is interesting mainly for its first line, which uses an ordinal index: For Each row In dtc(1).Rows Since you don t rename the second data table (you could do so with its TableName property), it is better to use the index rather than the name (customers1), since a change to the name in the Fill() call would require you to change it here, an unlikely thing to remember to do, if the case ever arises.
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Comparing FilterSort to PopDataSet
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In the first example, PopDataSet (Listing 13-1), you saw how simple it is to get data into a dataset. The second example, FilterSort (Listing 13-2), was just a variation, demonstrating how multiple result sets are handled and how to filter and sort data tables. However, the two programs have one major difference. Did you notice it FilterSort doesn t explicitly open a connection! In fact, it s the first (but won t be the last) program you ve written that doesn t. Why doesn t it The answer is simple but very important. The Fill method automatically opens a connection if it s not open when Fill() is called. It then closes the connection after filling the dataset. However, if a connection is open when Fill() is called, it uses that connection and doesn t close it afterward. So, although datasets are completely independent of databases (and connections), just because you re using a dataset doesn t mean you re running disconnected from a database. If you want to run disconnected, use datasets, but don t open connections before filling them (or, if a connection is open, close it first). Datasets in themselves don t imply either connected or disconnected operations. You leave the standard conn.Close(); in the finally block. Since Close() can be called without error on a closed connection, it presents no problems if called unnecessarily, but it definitely guarantees that the connection will be closed, whatever may happen in the try block.
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CHAPTER 13 s USING DATASETS AND DATA ADAPTERS
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s Note If you want to prove this for yourself, simply open the connection in FilterSort before calling
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Fill() and then display the value of the connection s State property. It will be Open. Comment out the Open() call, and run it again. State will be closed.
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Using Data Views
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In the previous example, you saw how to dynamically filter and sort data in a data table using the Select method. However, ADO.NET has another approach for doing much the same thing and more: data views. A data view (an instance of class System.Data.DataView) enables you to create dynamic views of the data stored in an underlying data table, reflecting all the changes made to its content and its ordering. This differs from the Select method, which returns an array of data rows whose contents reflect the changes to data values but not the data ordering.
s Note A data view is a dynamic representation of the contents of a data table. Like a SQL view, it doesn t
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